Hitherto, a sheet finisher is known which is disposed downstream of an image forming apparatus such as a copier, a printer, or a multi-function peripheral (MFP) and performs finishing such as punching or stitching on printed sheets.
Recently, functions of a sheet folding apparatus has been diversified, and the sheet folding apparatus (the sheet finisher) has been developed suggested which has, in addition to the punching and stitching functions, a folding function of folding a part of a sheet and a saddle-stitching and folding function of stitching the center of a sheet with staples and then folding the sheet at the center (see JP-A-2004-106991, JP-A-2003-182928, etc.).
The sheet folding apparatus having the saddle-stitching and folding function can form a booklet (bind a book) from plural printed sheets.
In the saddle-stitching and folding suggested hitherto, the center of sheets is stitched with staples or the like and then the stitched portion is creased and folded by a pair of rollers called folding rollers. At this time, a plate-like member called a folding blade is applied to the stitched portion of the sheet bundle and is pushed into a nip of the folding roller pair to crease the sheet bundle.
However, a time when the folded portion of the sheet bundle is pressed by the nip of the folding rollers is short and the entire folded portion is simultaneously pressed by the nip of the folding rollers. Accordingly, the pressure is dispersed to the entire fold. Thus, the fold formed by the folding rollers is a fold to which a sufficient pressure is not applied. Particularly, when the number of sheets is large or when the sheet bundle includes a thick sheet, an incomplete fold is often formed.
In order to deal with this problem, JP-A-2004-106991 or JP-A-2003-182928 discloses a technique of separately providing a roller called a fold reinforcing roller and reinforcing the fold formed by the folding rollers with the fold reinforcing roller.
In the technique disclosed in JP-A-2004-106991 or JP-A-2003-182928, the sheet bundle pushed out from the folding rollers is temporarily stopped on a guide plate and the fold reinforcing roller is made to move along the fold while applying a pressure from above to the fold of the sheet bundle. The fold nipped between the guide plate and the fold reinforcing roller is reinforced by the pressure generated between the guide plate and the fold reinforcing roller.
In addition, JP-A-2003-182928 discloses a technique of making a moving speed of the fold reinforcing roller variable depending on the number of sheets to be processed.
However, in the technique disclosed in JP-A-2004-106991 or JP-A-2003-182928, since the pressure is applied to the fold between the fold reinforcing roller and the plane guide plate, it is anticipated that the pressing force of the fold reinforcing roller is diffused by the plane guide plate and thus the pressure to reinforce the fold is not effectively applied to the fold.
A method is conceivable in which a fold in the nip of a pair of fold reinforcing rollers is reinforced by allowing the pair of fold reinforcing rollers to move along the fold with the fold interposed therebetween while applying a pressure to the nip. In this method, since the force of pressing the pair of fold reinforcing rollers to each other can be concentrated on one point of the nip, the high pressure can be generated at the nip, thereby more effectively reinforcing the fold.
Specifically, one fold reinforcing roller (first roller) of the pair can be made to freely rotate in a state where the position is fixed in the thickness direction of a sheet bundle. Meanwhile, the other fold reinforcing roller (second roller) of the pair can be made to freely rotate similarly to the first roller, and can be made to move in the thickness direction while applying an urging force by an elastic member such as a spring in the thickness direction of the sheet bundle.
By applying a strong urging force to the first and second rollers, it is possible to form an excellent fold sharp and not unfolded again.
However, since the urging force applied to the first and second rollers is strong, an impact sound not negligible is generated when the first and second rollers come in contact with each other in an area not having the sheet bundle or when the first and second rollers come in contact with each other in a state where no sheet bundle exists such as in an initial operation.